In the extraction of lead from lead ores by the sinter process, lead sinter is smelted in a blast furnace to produce bullion and slag which are separated externally to the blast furnace in a forehearth. Lead bullion from the forehearth is collected in cooling pots and then transferred to drossing kettles. Copper compounds with other impurities rejected from solution during cooling float to the surface in the drossing kettle. Coke, sawdust, and the like may be added to aid the formation of a dry powdery dross which is skimmed from the surface of the cooling lead. Sulphur may be added to remove final traces of copper from bullion.
Constituents of the skimmed off lead dross include about 20% by weight of lead sulphide, about 20% by weight of copper sulphide predominently as chalcocite, and about 50% by weight of entrained metallic lead metal. Minor amounts of antimony, arsenic and other impurities are also present. Typical assays for various types of dross are shown in Table I. It can be seen that it is not unusual for a dross of the type under discussion to have a lead assay of the order of 70% and a copper assay of from about 5% to about 35%.
Various methods have been proposed with the objective of recovering lead from dross. These include pyrometallurgical methods and hydroextractive methods. Of the pyrometallurgical methods U.S. Pat. Nos. 2,217,981 and 4,033,761 each describe processes in which lead bullion entrained in the dross is released but neither process enables recovery of bound lead present in the dross as lead sulphide, nor does either permit the copper to be recovered readily by virtue that the copper matte retains a high residual lead assay.
U.S. Pat. No. 4,333,763 provides a process which enables the copper constituents to be recovered with a sufficiently low lead content to be suitable for shipment to a copper refinery. In that process a pool of molten lead metal is first established and then metallic alkali metal, for example sodium metal, is incorporated in the lead pool. The lead sulphide bearing dross is added to the molten lead and alkali metal with stirring at a temperature not above 650.degree. C. The alkali metal reacts with the lead sulphide to reduce the combined lead to metallic lead and subsequently a matte phase comprising sulphur compounds of the alkali metal together with copper is separated from the molten lead. The alkali metal sulphides may be leached from the matte phase. As the process permits the handling of recovered copper matte in a conventional copper refining process, the process has considerable economic advantages in comparison with earlier processes. However the requirement for metallic sodium renders the process expensive to conduct, especially at locations remote from sodium manufacture.
An object of the present invention was the development of a pyrometallurgical process which avoids the disadvantages of the process described in U.S. Pat. No. 4,333,763 and which in preferred embodiments would enable the copper constituents to be recovered sufficiently free from lead constituents to be able to be processed in a copper smelter with little or negligible increase in contamination.